The present invention relates to an optical information recording and reproducing method and apparatus by which recording or reproducing of information is effected by irradiating a moving recording medium with focused laser to light, and in particular to such an apparatus having a fixed part and a movable part and being provided with split optics effecting recording and reproducing of information while moving the movable part.
As apparatuses for recording information by irradiating a recording medium with light, it has been known that (1) in an optical disk apparatus, information to be recorded is recorded in the form of uneven pits, (2) in an optomagnetic disk apparatus, information to be recorded is recorded while making it correspond to a medium, (3) in a phase change-type optical disk apparatus, reflectivity of light is varied at parts on a recording medium irradiated with light, etc. In many optical information recording and reproducing apparatuses, light for recording and reproducing information is provided by semiconductor laser devices in conjunction with optical or optomagnetic recording media.
A light beam emitted by a semiconductor laser device is collimated and focused on a small spot formed on a track of the recording medium. An optical head comprises the semiconductor laser device, the optics for collimating the light beam, the optics for irradiating the light beam to the small spot on the recording medium, etc., and is commonly housed in a casing. Accordingly position control for the small spot has often been performed by moving the optical head (refer to an article entitled "Ten billion bits on a disk", IEEE Spectrum AUGUST 1979, Pages 26-33, by K. Bulthuis et al.). However, in order to realize a high precision optical information recording and reproducing apparatus by moving the whole optical head with high speed, it is required to modify and downsize the optical head.
As to a type for moving the optical head with high speed, a split-type optical head has been proposed. In the split-type optical head, part of the optical head leading the light beam into the recording medium is split into a fixed part and a movable part. By this construction, the small, lightweight movable part is only moved to project the small spot on the recording medium, allowing correction of focusing deviation, correction of track position, and also information retrieval with high speed.
A split-type optical head has been described in detail in an article entitled "CMF Actuator and High Speed Access System", National Technical Report Vol. 35 No. 2, Apr. 1989; Pages 165-171, by Mitsuroh Moriya et al., and in an article entitled "High Speed Accessing using Split Optical Head" (SPIE Vol. 1078 Optical Data Storage Topical Meeting (1989); Pages 239-243) by K. Koumura et al.
However, such an optical head has a problem in that feedback noise of the semiconductor laser device is caused, in addition to a problem of down-sizing and reduction in weight. That is, a reflected part of the light irradiated from the semiconductor laser device to the recording medium again returns to the semiconductor laser device, which causes laser noise.
FIG. 4 shows an example of a conventional optical information recording and reproducing apparatus having a semiconductor laser device and a split-type optical head. A semiconductor laser device 1 emits a diverging linearly polarized light beam. The diverging light beam is transformed into a parallel light beam by collimating optics consisting of e.g. a collimating lens 5. The parallel light beam is passed through a polarized beam splitter 4. Then it passes through a quarter-wavelength plate 8 to be converted into a circularly polarized beam, and is bent upwardly by a mirror 12 and focused on a recording surface 14 of the recording medium 2 by an objective lens 6. The light beam irradiated on the recording surface 14 is reflected to pass back through through the objective lens 6 and to be reflected by the mirror 12 back through the quarter-wavelength plate 8. The reflected light beam thus becomes linearly polarized light whose polarization direction is perpendicular to the polarization direction of the original light beam emitted by the semiconductor laser device 1, enters the polarized beam splitter 4 and is reflected thereby to be focused by a detection lens 7 on a detector 3, where it is converted into an electric signal.
Since optical properties such as stability of lateral mode, small astigmatism, etc. are required for the laser used for the optical information recording and reproducing apparatus, a device having single modes both for lateral mode and transversal mode is used therefor. For this reason, strong coherency as well as laser noise are excited by returning light. For example, in an apparatus detecting presence or absence of light reflected by the recording medium for carrying out reproduction of information, since a quarter-wavelength plate is inserted in the optical path, it is possible to deflect most of the reflected light to the detecting system side and not to the light source side. However, a small part of the reflected light from the recording medium is fed back to the semiconductor laser device because of birefringence of a substrate of the recording medium and the characteristic fluctuations of optical parts, etc.
On the other hand, in an optomagnetic disk device, since variation in the direction of magnetization of the information recording medium is detected as rotation of the polarization plane, the quarter-wavelength plate cannot be inserted in the optical path, and thus several tens of % of the reflected light from the recording medium is fed back to the semiconductor laser device. In consideration of the reflectivity of the recording medium and the efficiency of the optical system, several % of the light emitted by the semiconductor laser device is fed back thereto as returning light. When the returning light amount exceeds several %, laser noise causes hindrance of recording and reproduction.
As to a countermeasure against the laser noise, the laser spectrum is converted into multi-mode by superposing a high frequency current on a current for driving the semiconductor laser to thereby reduce the coherency of the laser light, reducing laser noise caused by the returning light. Such a method is described in JP-B-59-9086, U.S. Pat. No. 4,712,218, JP-A-59210541. Among them, in JP-A-59-210541, assuming that the light velocity is c, the optical length between the semiconductor laser device 1 and the recording medium 2 is L, and the modulation frequency at which the laser noise becomes minimum is f, the following equation results from the above relationship, 2L/C=1/2f. According to the equation, an optical length up to the recording surface of the recording medium has been determined from the frequency of the high frequency current and the light emitting point of the semiconductor laser device.